After causing chickenpox in 4 million children annually, varicella zoster virus (VZV) becomes latent in cranial nerve, dorsal root and autonomic nervous system ganglia along the entire neuraxis. Decades later, virus reactivates, mostly in the elderly, producing zoster (pain and rash restricted to 1-3 dermatomes) in 600,000 to 1 million Americans yearly. Persistent VZV infection also causes myelitis and vasculopathy, and possibly postherpetic neuralgia (PHN), underlining the importance of understanding how VZV reactivates, and in particular, the mechanism(s) of VZV gene expression during latency. We were the first to show that: VZV is latent in human ganglia;VZV is present in neurons of ganglia in >90% of normal adults;the abundance of latent VZV DNA is highly variable;latent VZV DNA exists as a circular episome;and at least 5 VZV transcripts (ORFs 21, 29, 62, 63 and 66) as well as VZV gene 63 protein, are expressed during latency. During 10 years of MERIT award funding, our findings on prevalence of VZV transcripts, and VZV and simian varicella virus (SW) ORFs 63, and development of a monkey model of varicella latency provided a rationale for our hypotheses that: (1) an exact virologic definition of VZV latency is required in steering future research to understand the role of VZV genes in establishment, maintenance of, and reactivation from latency;and (2) that varicella ORF 63 regulates virus gene expression. Thus, our specific aims will: (1) determine the prevalence of all latently expressed VZV genes, thus defining latency at a molecular level. Because VZV ORF 63 appears to be the most abundant and prevalent transcript, and SW ORF 63 downregulates activation of the SW 21 promoter by SW ORF 62 in non-neuronal cells, but not neurons;we will: (2) determine the effect of VZV ORF 63 expression on regulation of virus gene expression by VZV ORF 62. We will unequivocally identify the exact region on the VZV ORF 63 protein that influences ORF 62 function in a cell type-specific fashion, which, will help our understanding of lytic vs. latent infection. Finally, we will: (3) determine if SW ORF 63 expression is essential for viral replication in cultured cells and infection in monkeys. Understanding latency will lead to approaches designed to prevent the cascade of events leading to virus reactivation, a cause of serious neurologic disease in the rapidly increasing elderly and immunocompromised populations.